The reaction of carbon monoxide and hydrogen (so-called synthesis gas) to form methane and higher carbon number hydrocarbons has been described in the prior art. Nowhere, however, is there a description in the prior art of how to control or effect the distribution of the C.sub.2 + hydrocarbons in the product so as to peak or maximize the production of C.sub.2 to C.sub.6 hydrocarbons, i.e. LPG hydrocarbons rather than the higher carbon number gasoline and diesel range type hydrocarbons. A catalyst must be employed, and the prior art catalysts include iron, nickel and ruthenium, among others. U.S. Pat. No. 3,974,483, issued Mar. 30, 1976 in the names of T. P. Kobylinski and H. E. Swift, teaches the use of certain layered complex metal silicates, typified by nickel chrysotile, for the reaction of CO and hydrogen to produce solely methane. It is to be noted that the nickel chrysotile is selective to the formation of methane. Volume IV of "Catalysis" by P. H. Emmett and published by Reinhold Publishing Co., N.Y. (1956) (see pp. 56 et seq.) teaches the use of nickel plus thoria as a catalyst for the production of gasoline range hydrocarbons from synthesis gas. The same reference teaches the use of cobalt plus thoria for the same purpose. It is also well known that nickel alone on kieselguhr produces methane as substantially the sole product by the reaction of synthesis gas in the presence of the nickel catalyst. Pollitzer et al., for example, in U.S. Pat. No. 3,361,535, teach the removal of small amounts of CO from a hydrogen stream by a methanation step embodying the reaction: EQU CO + 3H.sub.2 = CH.sub.4 + H.sub.2 O
(col. 4, lines 10-13). Pollitzer et al. teach that various hydrogenation or synthesis catalysts comprising iron, nickel, or more particularly metals of the Iron and Platinum Groups may be used to effect the CO methanation; however, according to Pollitzer et al., a nickel containing catalyst is preferred to obtain the maximum conversion to methane (Col. 4, line 20). Various promoting agents or metallic hydrogenating components are suggested by Pollitzer et al. to be used in combination with nickel or to be incorporated with the support to assist in the methanation step. With regard to the addition of "silica, zirconia, thoria, ceria, beryllia, vanadia, etc." to alumina, Pollitzer et al. teach that "certain" of these "may" have a catalytic or promotional effect but then only in assisting the methanation step. There are no suggestions or teachings in Pollitizer et al. that any hydrocarbons other than methane are intended to be or would be produced using any of their suggested catalytic materials. More importantly, the Pollitzer et al. invention relates to the removal of less than one percent carbon oxide from a stream containing 90 % or more hydrogen (Col. 5, lines 45-46). Thus the hydrogen to CO ratio is extremely high, and, as will be shown with the data below, as the hydrogen to CO ratio increases, the amount of C.sub.2 plus hydrocarbons dramatically decreases. Even if, for the sake of argument only, the Pollitzer et al. reference was considered as teaching or suggesting the production of higher carbon number products than methane, there is no suggestion regarding (i) the distribution of such hydrocarbon products or (ii) which of the many materials taught as having a "catalytic or promotional" effect would produce predominantly hydrocarbon products having from 2 to 6 carbon atoms per molecule.
U.S. Pat. No. 2,517,035 to Sensel et al. teaches the use of lanthanum oxide in combination with magnesia as a promoter for a cobalt containing catalyst. However, Sensel et al. provide no indication in their specification as to the breakdown of the types of hydrocarbon products which they have obtained. That is, Sensel et al. simply indicate that the product is a C.sub.3 + liquid hydrocarbon. Cobalt catalysts are known to produce hydrocarbons in the 4-8 and higher carbon atom range. (H. H. Storch et al., "The Fischer Tropsch and Related Syntheses", John Wiley & Sons, N.Y. (1951), p. 151).) Manganese is also known to be effective as a promoter for nickel catalysts for the production of gasoline range hydrocarbons (see again P. H. Emmett's book referred to above, p. 58). Nickel, on the other hand, per se, is not known as a catalyst to produce gasoline range hydrocarbons, but rather is employed to produce methane alone.
The present invention uses a rare earth promoted nickel chrysotile catalyst not only to accelerate the hydrogenation reaction but also to selectively produce low molecular weight hydrocarbons. By adding rare earths to the nickel chrysotile catalyst, it is possible to obtain significant amounts of C.sub.2 to C.sub.6 hydrocarbons in addition to methane. Further, there are only very small amounts of C.sub.7 + products made.
An improved synthesis reaction is accomplished in accordance with the invention by contacting CO, CO.sub.2, or mixtures of these carbon oxides and hydrogen wherein the molar ratio of the hydrogen to carbon oxides is from 1:1 to 4:1 under synthesis conditions including a temperature from 300.degree. to 500.degree. F. (149.degree. to 260.degree. C.) in the presence of a catalyst consisting essentially of a rare earth promoted crystalline layered complex metal silicate characterized as having repeating units having the structural formula: EQU [(1-x)Ni.sup.a + xRu.sup.b ].sub.n (OH).sub.4 Si.sub.2 O.sub.5. wH.sub.2 O
where x is a number from 0 to 1, this number expressing the atomic fraction of the metals mickel and ruthenium; a is the valence of nickel; b is the valence of ruthenium; n is a number equal in value to that defined by the ratio EQU 6/[a (1-x) + bx]
and w is a number ranging from 0 to 4. The hydrocarbon product on a methane-free basis is found to contain at least 60 mole percent of low molecular weight hydrocarbons having from 2 to 6 carbon atoms per molecule.
The improved hydrogenation catalyst for use in the process of this invention is a known layered complex metal silicate wherein the metal is selected from nickel, ruthenium, or mixtures of these metals. These layered complex metal silicates and their methods of preparation are described, for example, in U.S. Pat. No. 3,729,429 to Robson, issued Apr. 24, 1973. The specification of the Robson patent is incorporated herein by reference for the purpose of providing a fuller description of the metal silicates and their method of preparation. It is realized that the materials described by Robson encompass many complex metal silicates while only the nickel and ruthenium or mixed nickel-ruthenium complex metal silicates are claimed in this specification as useful materials to promote the hydrogenation reaction. Robson in his specification describes his metal silicates as useful catalytic agents in hydrocarbon conversion reactions. Illustrative of such reactions are aromatization, isomerization, hydroisomerization, cracking, hydrocracking, polymerization, alkylation, dealkylation, hydrogenation and dehydrogenation, desulfurization, denitrogenation and reforming (see Col. 3, lines 14-18 of the Robson U.S. Pat. No. 3,729,429). Nowhere does Robson teach or indicate that his materials, especially the nickel or ruthenium forms, are useful for the synthesis of low molecular weight hydrocarbons.
More specifically, the catalyst used to promote the desired reaction in accordance with this invention is a rare earth promoted crystalline layered complex metal silicate composition characterized as having repeating units having the structural formula EQU [(1-x)Ni.sup.a + xRu.sup.b ].sub.n (OH).sub.4 Si.sub.2 O.sub.5.wH.sub.2 O
where x is a number from 0 to 1, this number expressing the atomic fraction of the metals nickel and ruthenium; a is the valence of nickel; b is the valence of ruthenium; n is a number equal in value to that defined by the ratio EQU 6/[a(1-x) + bx]
and w is a number ranging from 0 to 4.
The preferred metal silicate is where x in the above formula equals 0. The resulting material is a nickel chrysotile, and naturally occurring nickel chrysotile is known as garnierite.
Thus either naturally occurring nickel chrysotile can be employed to promote the subject reaction, or, more preferably, a synthetically prepared nickel chrysotile can be employed. One suitable method of preparing the catalysts of this invention is, as noted above, by the technique of Robson in U.S. Pat. No. 3,729,429. As noted by Robson at the top of Column 4, Ni.sub.3 (OH).sub.4 Si.sub.2 O.sub.5 (garnierite) is found in nature in the form of tubes. Robson acknowledges that synthetic garnierite has been prepared by prior art workers. The nickel chrysotile used in the working examples later in this specification, however, was prepared in accordance with the techniques of Robson, and thus the Robson technique is the preferred, although not the only, method of preparing the catalyst for use in the subject invention. In general, this process is to initially synthesize a gel be coprecipitation of the metal oxide or hydroxide with hydrous silica gel in an alkaline medium wherein the pH is above 10, preferably about 12 to 14. The composition of the metal hydroxide layer of the crystal is fixed by selecting the concentration of the nickel and ruthenium salts to vary the ratio of nickel to ruthenium as desired. Any water soluble nickel or ruthenium salts can be employed. After the desired gel is produced, it is heated from approximately 200.degree. to 350.degree. C., preferably 250.degree. to 275.degree. C., so that the chrysotile product is crystallized from the synthesis gel with rejection of excess water and soluble salts which are removed by filtration and washing. The complex metal silicates as defined above are generally prepared synthetically in hydrated form and are then converted to a dehydrated form by heating prior to use or in situ operation. Since the dehydration reaction is reversible and since water is produced during the hydrogenation reaction, the exact degree of hydration of the catalyst as the reaction proceeds is not known. Thus w in the above formula is defined as ranging from 0 to 4 to indicate that the degree of hydration of the catalyst may vary.
The nickel, ruthenium, or mixed nickel-ruthenium chrysotiles are dried to remove surface moisture and may or may not be dehydrated in whole or in part by calcination prior to use. The catalyst also, preferably, undergoes a mild prereduction before use. Calcination is not essential, nor is prereduction with a gas such as hydrogen essential, although varying degrees of calcination and/or prereduction may occur. Since the hydrogenation reaction is operated at elevated temperatures and in the presence of reducing gases, dehydration and reduction of the catalyst will occur. Precalcination can suitably occur at temperatures of 300.degree. to 500.degree. C. for 2 to 10 hours. Prereduction using a gas such as H.sub.2 at flow rates of 50 to 500 cc/min can also suitably occur at temperatures of 300.degree. to 500.degree. C. for 2 to 10 hours.
The metal silicate, preferably nickel chrysotile, is promoted using rare earth metals or rare earth metal oxides for the conversion of carbon monoxide and hydrogen to low molecular weight hydrocarbons, i.e. hydrocarbons having 1 to 6 carbon atoms. The product consists of a hydrocarbon portion and a non-hydrocarbon portion. The non-hydrocarbon product consists of H.sub.2 O, H.sub.2 and perhaps some unreacted CO or CO.sub.2, while, of course, the "hydrocarbon product" consists mostly of paraffins including methane and higher carbon number paraffins. Typically the methane-free hydrocarbon product contains at least 60 mole percent of C.sub.2 to C.sub.6 paraffins, usually at least 70 mole percent C.sub.2 to C.sub.6 paraffins, and optimally at least 90 mole percent C.sub.2 to C.sub.6 paraffins. By "promotion" is meant the activity of the metal silicate (as measured by conversion) increases together with an increase in selectivity to the production of C.sub.2 to C.sub.6 hydrocarbons. The rare earth metal or rare earth metal oxide is dispersed uniformly in the nickel chrysotile by any suitable means, for example, by physical admixture; or, more usually and preferably, by deposition from a solution, preferably aqueous, of a suitable salt, such as a rare earth metal nitrate. Such rare earths are difficult to reduce; they are usually present in the oxide form. Rare earth metals including the oxide form and mixtures of these metals which can be used include metals having the following atomic numbers: 21, 39, 57 to 72, and 90. Mixtures of the rare earths are preferred. A promoting amount of rare earth metal oxide is employed and this is usually from 0.5 to 25 weight percent, preferably 1.5 to 5 weight percent of the total catalyst.
The charge stock for the synthesis reaction comprises hydrogen and at least one carbon oxide selected from the group consisting of CO and CO.sub.2 wherein the molar ratio of hydrogen to combined carbon oxides is from 1:1 to 4:1. Preferably the hydrogen to combined carbon oxides molar ratio is from about 1:1 to 2.5:1 more preferably from 1:1 to 2.2:1; and most preferably from 1:1 to 2:1. As will be shown below, as the H.sub.2 to CO ratio increases, the amount of C.sub.2 + hydrocarbons decreases. Since the process of this invention is directed to the production of LPG type paraffins, the lower hydrogen to combined carbon oxide ratios are preferred.
Ideally the synthesis reaction proceeds in accordance with Equation 1 when CO is the reactive carbon oxide employed.
Equation 1 EQU (2n + 1)H.sub.2 + nCO .revreaction. CnH.sub.2n+2 +nH.sub.2 O
for instance, when n equals one,
Equation 1(a) EQU 3 H.sub.2 + CO .revreaction. CH.sub.4 + H.sub.2 O
or when n equals three,
Equation 1(b) EQU 7H.sub.2 + 3CO .revreaction. C.sub.3 H.sub.8 + 3H.sub.2 O
referring to Equations 1, 1(a) and 1(b), stoichiometry indicates that the minimum hydrogen to CO mole ratio is 2:1. Hydrogen to CO ratios as low as 1:1 can be used, as noted previously, but reduced reaction efficiency results. Higher hydrogen to CO ratios, above 2:1, tend to discourage side reactions such as the decomposition of CO to form carbon (coke), but the yield of C.sub.2 plus hydrocarbon products also decreases at the higher H.sub.2 to CO ratios.
If the hydrogen to CO mole ratio is below about 2:1, a secondary water-gas shift reaction can occur as shown by Equation 2:
Equation 2 EQU CO + H.sub.2 O .revreaction. CO.sub.2 + H.sub.2
methane and CO.sub.2 may also be produced as shown in Equation 3:
Equation 3 EQU 2CO + 2H.sub.2 .revreaction. CH.sub.4 + CO.sub.2
if CO.sub.2 is present either initially or via Equations 2 and 3, methane and C.sub.2 and higher hydrocarbons can also be produced as shown in Equation 4:
Equation 4 EQU (3n + 1)H.sub.2 + nCO.sub.2 .revreaction. C.sub.n H.sub.2n+2 + 2nH.sub.2 O
for instance, when n equals one,
Equation 4(a) EQU 4H.sub.2 + CO.sub.2 .revreaction. CH.sub.4 + 2H.sub.2 O
and when n equals three,
Equation 4(b) EQU 10 H.sub.2 + 3CO.sub.2 .revreaction. C.sub.3 H.sub.8 + 6H.sub.2 O
since hydrogen is the more expensive component of the charge stock, it is naturally preferred to keep the CO.sub.2 content of the charge stock as low as possible, albeit a charge consisting essentially of CO.sub.2 can be employed if desired. The higher molar ratios of H.sub.2 to carbon oxides, as noted previously, can be used to a limit of 4:1 despite the relative high cost of hydrogen.
The charge stock for the reaction of synthesis gas of this invention can, of course, be obtained from any suitable source well known to those in the art. For example, if pipeline gas (SNG and LPG, i.e. methane and C.sub.2 -C.sub.6 hydrocarbons, respectively) is the desired final product, the charge stock for the synthesis reaction can be derived from the gasification of coal with steam and oxygen. Initial coal gasification product streams are too low in hydrogen and contain undesirable impurities, especially sulfur compounds which tend to deactivate the catalysts of this invention. A typical coal gasification product on a water-free basis contains about 29% CO.sub.2 ; 19% CO; 38% H.sub.2 ; 13% methane; and small amounts of H.sub.2 S and nitrogen. Normally these gases are purified to remove sulfur (to less than 1 ppm), and the gases are then subjected to a water-gas shift reaction (Equation 2) to increase the H.sub.2 and thus give a product gas stream which is suitable as a charge stock to a synthesis reactor, e.g. where the H.sub.2 to combined carbon oxides is from 1:1 to 4:1.
Diluent gases such as nitrogen or steam can also be present in the charge stock and the amount of inert material in the charge must be balanced by its usefulness as a heat sink versus the reduced space-time yields of products which are achieved because of the presence of the diluent. In one preferred embodiment of the invention, recycle product (after removal of LPG) consisting primarily of methane is used as the diluent heat sink.
The synthesis reaction occurs by contacting the charge stock with the desired catalyst under synthesis conditions including a temperature from 300.degree. to 500.degree. F. (149.degree. to 260.degree. C.), preferably from 350.degree. to 450.degree. F. (177.degree. to 232.degree. C.), and most preferably from 375.degree. to 425.degree. F. (190.degree. to 218.degree. C.). It is also desirable to utilize an adiabatic reactor such as are commonly employed in Fischer-Tropsch or methanol synthesis. The reaction is highly exothermic and, as noted previously, it is preferred to recycle a portion of the LPG-free product to serve as a heat sink.
The charge stock is usually preheated to a temperature of 300.degree. to 450.degree. F. (149.degree. to 232.degree. C.). This preheated gas is then contacted with the rare earth promoted metal chrysotile catalyst of this invention under synthesis conditions. By "under synthesis conditions" is meant under conditions of temperature, pressure and space velocity for the charge stock whereby the desired hydrocarbon products are produced by the reaction of H.sub.2 and CO and/or CO.sub.2. Such synthesis conditions, except for temperature, are not critical and are well known to those in this art. The temperature of the reaction, as noted, can be at least 300.degree. F. (149.degree. C.), preferably at least 350.degree. F. (177.degree. C.) and can be as high as 500.degree. F. (260.degree. C.) but, is preferably not above 450.degree. F. (232.degree. C.). The gaseous hourly space velocity (GHSV) can suitably be from 1 to 100,000 volumes of gas (total gas including recycle product) per volume of reactor per hour, preferably 100 to 10,000 v/v/hr, and most preferably 200 to 2000 v/v/hr. The reaction pressure is normally atmospheric to 1000 psi; however, increased pressures of up to 10,000 psi or more can be employed. An upflow fixed bed operation using extrudates, pellets or other suitably shaped and sized catalyst particles can be employed, but obviously, downflow operation or other types of catalyst beds, e.g. fluid beds, can also be employed.
The product from the reactor differs in composition from the charge stock by an increase in the concentration of methane, higher hydrocarbons and water, and a decrease in the content of hydrogen and carbon oxides. A portion of the product, usually free of LPG, is suitably recycled for admixture with the preheated charge stock to serve as a heat sink in the reactor. The recycle to feed gas volume ratio is usually about 2:1 but can be from 5:1 to 10:1 or more as desired.